The role of Mott-Schottky heterojunctions in Ag-Ag8SnS6 as counter electrodes in dye-sensitized solar cells

Electrodeposited PPy@TiO2 and PEDOT@TiO2 Counter Electrodes for [Co(bpy)3]2+/3+ Redox Mediator-Based Dye-Sensitized Solar Cells

The main goal of this work is to enhance the catalytic performance of PPy and PEDOT films toward the Co2+/Co3+ redox couple. PPy and PEDOT films were electrodeposited separately on a porous TiO2 template to assess their suitability as alternative catalysts in dye-sensitized solar cells (DSSC) based on the [Co(bpy)3]2+/3+ redox shuttle. The obtained PPy@TiO2 and PEDOT@TiO2 counter electrodes displayed much rougher surfaces. Electrochemical studies indicate the superior catalytic activity of both the electrodeposited electrodes toward Co3+ reduction, as indicated by lower charge transfer resistance than that of pristine films and even that of Pt electrodes. Therefore, the fabricated DSSC devices with these counter electrodes achieved higher power conversion efficiencies compared to cells with pristine PPy and PEDOT counter electrodes, or even with a Pt counter electrode. Interestingly, the assembled DSSC device with a PEDOT@TiO2 counter electrode displayed the highest performance among all with a power conversion efficiency of 6.62%, which is better than that obtained by the device with a Pt electrode (6.07%).

Ameliorating Cu2+ reduction in microbial fuel cell with Z-scheme BiFeO3 decorated on flower-like ZnO composite photocathode

BiFeO₃ nanoparticle decorated on flower-like ZnO (BiFeO₃/ZnO) was fabricated through a facile hydrothermal-reflux combined method. This material was utilized as a composite photocathode for the first time in microbial fuel cell (MFC) to reduce the copper ion (Cu²⁺) and power generation concomitantly. The resultant BiFeO₃/ZnO-based MFC displayed distinct photoelectrocatalytic activities when different weight percentages (wt%) BiFeO₃ were used. The 3 wt% BiFeO₃/ZnO MFC achieved the maximum power density of 1.301 W m^-2 in the catholyte contained 200 mg L^-1 of Cu²⁺ and the power density was greatly higher than those pure ZnO and pure BiFeO₃ photocathodes. Meanwhile, the MFC exhibited 90.7% removal of Cu²⁺ within 6 h under sunlight exposure at catholyte pH 4. The addition of BiFeO₃ nanoparticles not only manifested outstanding capability in harvesting visible light, but also facilitated the formation of Z-scheme BiFeO₃/ZnO heterojunction structure to induce the charge carrier transfer along with enhanced redox abilities for the cathodic reduction. The pronounced electrical output and Cu²⁺ reduction efficiencies can be realized through the synergistic cooperation between the bioanode and BiFeO₃/ZnO photocathode in the MFC. Furthermore, the developed BiFeO₃/ZnO composite presented a good stability and reusability of photoelectrocatalytic activity up to five cyclic runs.

A Low-Temperature Structural Transition in Canfieldite, Ag8SnS6, Single Crystals

Canfieldite, Ag₈SnS₆, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag₈SnS₆ of mass up to 1 g from a ternary Ag-Sn-S melt. On cooling from high temperature, Ag₈SnS₆ undergoes a known cubic (F4̅3m) to orthorhombic (Pna2₁) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5-300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn2₁ (a = 7.662 9(5) Å, b = 7.539 6(5) Å, c = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag₈SnSe₆ and Ag₈GeSe₆. The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag₈SnS₆ with its argyrodite analogues, Ag₈TQ₆ (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna2₁ arrangement. We support this picture by showing that the transition to the Pmn2₁ phase is avoided in Ge alloyed Ag₈Sn_1-xGe_xS₆ samples as well as in pure Ag₈GeS₆.

Direct synthesis of nanostructured silver antimony sulfide powders from metal xanthate precursors

Silver(I) ethylxanthate [AgS2COEt] (1) and antimony(III) ethylxanthate [Sb(S2COEt)3] (2) have been synthesised, characterised and used as precursors for the preparation of AgSbS2 powders and thin films using a solvent-free melt method and spin coating technique, respectively. The as-synthesized AgSbS2 powders were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The crystalline AgSbS2 powder was investigated using XRD, which shows that AgSbS2 has cuboargyrite as the dominant phase, which was also confirmed by Raman spectroscopy. SEM was also used to study the morphology of the resulting material which is potentially nanostructured. EDX spectra gives a clear indication of the presence of silver (Ag), antimony (Sb) and sulfur (S) in material, suggesting that decomposition is clean and produces high quality AgSbS2 crystalline powder, which is consistent with the XRD and Raman data. Electronic properties of AgSbS2 thin films deposited by spin coating show a p-type conductivity with measured carrier mobility of 81 cm2 V-1 s-1 and carrier concentration of 1.9 × 1015 cm-3. The findings of this study reveal a new bottom-up route to these compounds, which have potential application as absorber layers in solar cells.

Biomass-derived carbon boosted catalytic properties of tungsten-based nanohybrids for accelerating the triiodide reduction in dye-sensitized solar cells

Manganese tungstate (MnWO₄), zinc tungstate (ZnWO₄), and copper tungstate (CuWO₄) embedded biomass-derived carbon (MWO-C, ZWO-C, CWO-C) was synthesized by hydrothermal treatment and investigated as counter electrode (CE) catalysts to test electrochemical activity. Biomass-derived carbon was used as the shape controlling agent, which changed the morphology of MWO from spherical to spindle-like. Owing to the synergistic effect between tungsten-based bimetal oxides and biomass-derived carbon, the MWO-C, ZWO-C, and CWO-C catalysts exhibited enhanced electrochemical performance in dye-sensitized solar cells (DSSCs) system. The MWO-C, ZWO-C and CWO-C catalysts in DSSCs showed outstanding power conversion efficiency (PCE) of 7.33%, 7.61%, and 6.52%, respectively, as compared with 7.04% for Pt based devices. Biomass-derived carbon improves the catalytic properties of tungsten-based nanohybrids. The results showed that biomass-derived carbon-enhanced inorganic compound as CE catalysts are promising alternatives to Pt-based CE catalysts for energy conversion devices.

An all carbon dye sensitized solar cell: A sustainable and low-cost design for metal free wearable solar cell devices

Lightweight carbon electrodes are the new candidates for photovoltaic devices due to their temperature resistivity, ease of fabrication, and skin comfortability. Herein, a sustainable and facile strategy has been proposed for metal free all carbon dye sensitized solar cell (C-DSSC), assembled by stacking carbon front electrode (CFE) and carbon counter electrode (CCE). The CFE demonstrated adequate light transmittance (70-50%) while maintaining efficient photon absorption and charge separation mechanism due to dye coated TiO₂ nanorods (P25-R). The graphene dip coated carbon counter electrode (Gr@CCE) possesses remarkable electro catalytic activity towards I₃^-/I^- redox couple with low charge transfer resistance (R_CT = 0.79 Ω). The sustainable design of C-DSSC attained ~6 ± 0.5% efficiency with high photocurrent density of 18.835 mA. cm^-2. The superior performance of C-DSSC is accredited to its improved charge mobility, low internal resistance, and better interfacial electrode contact. The thickness of C-DSSC is ≤3 mm eliminates the need for rigid glass in DSSC.

Electrochemical investigation of uncapped AgBiS2 (schapbachite) synthesized using in situ melts of xanthate precursors

In this study, a facile and potentially scalable synthesis of AgBiS2 (schapbachite) using melts of metal xanthates is presented; AgBiS2 is both a significant mineral and a technologically important material. This ternary material was synthesized by a novel and low-cost solventless route using simple ethyl xanthate complexes of silver and bismuth. p-XRD analysis indicates that the synthesized ternary material is highly crystalline and belongs to the cubic phase (schapbachite). The electrochemical properties of the material were tested; the potential of the synthesized material for application in charge storage shows a high specific capacitance of 460 F g-1 at 2 mV s-1. A capacitance retention of 83% with a 100% coulombic efficiency was observed after 3000 cycles. The charge storage potential, analysed by fabricating actual symmetrical devices, shows a specific capacitance of 14 F g-1 at 2 mV s-1. An energy density of 26 W h kg-1 and a power density of 3.6 kW kg-1 were observed. Besides, the potential for the oxygen evolution reaction was also studied. An overpotential of 414 mV and a Tafel slope of 134 mV dec-1 were obtained for water oxidation. The fabrication of an electrolyzer cell using the synthesized material as the cathode indicates that a current of 10 mA cm-2 can be achieved at a potential of 1.63 V.

Wide spectral degradation of Norfloxacin by Ag@BiPO4/BiOBr/BiFeO3 nano-assembly: Elucidating the photocatalytic mechanism under different light sources

Metallic Ag deposited BiPO₄/BiOBr/BiFeO₃ ternary nano-hetero-structures were rationally designed and synthesized by a simple precipitation-wet impregnation-photo deposition method. The plasmonic junction possesses an excellent wide spectrum photo-response and makes best use of BiPO₄ which is otherwise a poor photocatalyst. Ag@BiPO₄/BiOBr/BiFeO₃ showed superior photocatalytic activity for degradation of norfloxacin (NFN) under visible, ultra-violet, near-infra-red and natural solar light. Especially catalyst APBF-3 (0.3 wt% Ag@BiPO₄/BiOBr/BiFeO₃) shows 98.1% degradation of NFN (20 mg/L) in 90 min under visible light and 99.1% in less than 45 min under UV exposure. Free radical scavenging experiments and electron spin resonance (ESR) results has been used for explanation of charge transfer, photocatalytic mechanism and role of radicals for binary, ternary and Ag deposited ternary junctions for UV and visible exposure. Metallic Ag in addition to its surface plasmon resonance helps in protection of high conduction band and valence band in the three semiconductors. A dual Z-scheme mechanism has been predicted by comparing with possibilities of double charge and vectorial charge transfer.

Visible photodegradation of ibuprofen and 2,4-D in simulated waste water using sustainable metal free-hybrids based on carbon nitride and biochar

Rational designing of metal-free carbon nitride based photocatalysts can lead to an excellent optical response and a higher photocatalytic activity driven by visible and solar light. This combines green photocatalytic technology with greener materials prepared by facile approaches for environmental remediation. Herein we report utilization of star photocatalyst g-C₃N₄ (CN) to form highly efficient hetero-assemblies along with acidified g-C₃N₄ (ACN), polyaniline (PANI), reduced graphene oxide (RGO) and biochar. By use of these organic semiconductors we synthesize g-C₃N₄/ACN/RGO@Biochar (GARB), g-C₃N₄/PANI/RGO@Biochar (GPRB) and ACN/PANI/RGO@Biochar (APRB) nano-assemblies with different optical response and band edge positions for a better charge flow and reduced recombination of carriers. These synthesized catalysts were used for visible light powered degradation of 2,4-Dichlorophenoxy acetic acid (2,4-D) and ibuprofen (IBN). APRB performs the best and degrades 99.7% and 98.4% of 2,4-D and IBN (20 mg L^-1) under Xe lamp exposure in 50 min and retention of high activity in natural sunlight. Optical analysis, photoelectrochemical response and radical quenching studies show both hydroxyl and superoxide radical anions as major reactive species and a Z-scheme photocatalytic mechanism. RGO acts as an electron mediator and protects higher positioned bands of PANI and ACN in APRB for a remarkable photocatalytic activity for a metal free material. The degradation pathway was analyzed by LC-MS analysis and 42% and 40% total organic carbon was removed in 2 h for 2,4-D and IBN degradation respectively. The toxicity of degraded products was analyzed by analyzing viability of human peripheral blood cells with retaining of 99.1% cells.

Facile charge transfer in fibrous PdPt bimetallic nanocube counter electrodes

Enhanced charge transfer is realized in DSSCs that utilize poriferous PdPt bimetallic nanocubes as the counter electrode.

High-Performance Photocatalytic Hydrogen Production and Degradation of Levofloxacin by Wide Spectrum-Responsive Ag/Fe3O4 Bridged SrTiO3/g-C3N4 Plasmonic Nanojunctions: Joint Effect of Ag and Fe3O4

Highly photoresponsive semiconductor photocatalysis for energy and environmental applications require judicious choice and optimization of semiconductor interfaces for wide spectral capabilities. This work aims at rational designing of highly active SrTiO₃/g-C₃N₄ junctions bridged with Ag/Fe₃O₄ nanoparticles for utilizing Z-scheme transfer and surface plasmon resonance effect of Ag augmented by iron oxide. The SrTiO₃/(Ag/Fe₃O₄)/g-C₃N₄ (SFC) catalyst was employed for photocatalytic hydrogen production and photodegradation of levofloxacin (LFC; 20 mg/L) under UV, visible, near infra-red, and natural solar light exhibiting high performance. Under visible light (<780 nm), SFC-3 sample (30 wt % g-C₃N₄ and 3% Ag/Fe₃O₄) shows a H₂ evolution of 2008 μmol g^-1 h^-1 which is ∼14 times that of bare g-C₃N₄. In addition, 99.3% removal of LFC was degraded in 90 min under visible light with retention of activity under sun. The inherent topological properties, complete, higher charge separation, and reduced recombination allowed this catalyst for a high photocatalytic response which was proved by UV-diffuse reflectance spectroscopy, photoluminescence, electrochemical impedance spectroscopy, and photocurrent response measurements. Scavenging experiments and electron spin resonance analysis reveal that the mechanism shifts from a dual charge transfer in case of binary junction to essential Z-scheme with incorporation of Ag/Fe₃O₄. Both ^•O₂^- and ^•OH are main active radicals in visible light, whereas ^•O₂^- majorly participate under UV. The synergistic effect of SrTiO₃, g-C₃N₄, and plasmon resonance of Ag/Fe₃O₄ not only improves light response and reduce recombination but also enhances the redox-ability of charge carriers. A H₂ production mechanism and LFC degradation pathway (degradation, defluorination, and hydrolysis) has been predicted. This work paves a way for development of photocatalysts working in practical conditions for pollution and energy issues.

Ag8SnS6: a new IR solar absorber material with a near optimal bandgap

We report the synthesis and photovoltaic properties of a new ternary solar absorber – Ag₈SnS₆ nanocrystals prepared by successive ionic layer adsorption reaction (SILAR) technique. The synthesized Ag₈SnS₆ nanocrystals have a bandgap E_g of 1.24–1.41 eV as revealed from UV-Vis and external quantum efficiency (EQE) measurements. Its photovoltaic properties were characterized by assembling a liquid-junction Ag₈SnS₆ sensitized solar cell for the first time. The best cell yielded a J_sc of 9.29 mA cm⁻², a V_oc of 0.23 V, an FF of 31.3% and a power conversion efficiency (PCE) of 0.64% under 100% incident light illumination using polysulfide electrolyte and Au counter electrode. The efficiency improved to 1.43% at a reduced light intensity of 10% sun. When the polysulfide was replaced by a cobalt electrolyte with a lower redox level, the V_oc increased to 0.54 V and PCE increased to 2.29% under 0.1 sun, a respectable efficiency for a new solar material. The EQE spectrum covers the spectral range of 300–1000 nm with a maximum EQE of 77% at λ = 600 nm. The near optimal E_g and the respectable photovoltaic performance suggest that Ag₈SnS₆ nanocrystals have potential to be an efficient IR solar absorber.

We report the synthesis and photovoltaic properties of a new ternary solar absorber – Ag₈SnS₆ nanocrystals prepared by successive ionic layer adsorption reaction (SILAR) technique.

CuFeS2 colloidal nanocrystals as an efficient electrocatalyst for dye sensitized solar cells

Cubic CuFeS₂ nanocrystals (NCs) have been obtained via a facile colloidal chemistry approach and they show remarkable catalytic activity in the reduction of I₃^-. Dye sensitized solar cells (DSSCs) with CuFeS₂ NCs as counter electrodes (CEs) display a power conversion efficiency of 8.10% comparable to that of a cell with Pt as the CE (7.74%) under the same conditions.

Encapsulating CoS2–CoSe2 heterostructured nanocrystals in N-doped carbon nanocubes as highly efficient counter electrodes for dye-sensitized solar cells

The CoS₂–CoSe₂@N-doped carbon nanocubes were synthesized through simultaneous sulfurization and selenization of polydopamine coated Prussian blue analogs.

Bismuth-based ternary nanowires as efficient electrocatalysts for dye sensitized solar cells

Bismuth-based ternary nanowires (NWs), including Bi₁₉S₂₇Br₃ and Bi₁₉S₂₇I₃, have been exploited as cost-effective and highly efficient Pt-free electrocatalysts in dye sensitized solar cells (DSSCs). Devices with them as counter electrodes (CEs) display a power conversion efficiency (PCE) of over 8.70%, superior to that of a cell with Pt as a CE (7.99% PCE) under the same conditions.

Biogenic synthesis of silver nanoparticle by using secondary metabolites from Pseudomonas aeruginosa DM1 and its anti-algal effect on Chlorella vulgaris and Chlorella pyrenoidosa

Biogenic synthesis of silver nanoparticles (AgNPs) using extracellular metabolites from the bacterium Pseudomonas aeruginosa DM1 offers an eco-friendly and sustainable way of metal nanoparticle synthesis. The present work highlights the biotransformation of silver nitrate solution into AgNP, mediated by extracellular secondary metabolite pyoverdine, a siderophore produced by P. aeruginosa. The bioreduction of silver ions into AgNPs by using pyoverdine was recorded in terms of Fourier transform infrared spectroscopy (FTIR) analysis and color change in the reaction mixture (AgNO₃ + pyoverdine) from pale yellow to dark brown with absorption maxima at 415 nm. The results of X-ray diffraction (XRD) analysis of AgNPs showed its crystalline face-centered cubic structure. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) pictures of AgNPs showed spherical morphology of AgNP in the range of 45-100 nm, with tendency of agglomerations. The energy-dispersive X-ray (EDX) analysis of particles provided strong signal of elemental silver with few minor peaks of other impurities. The present approach offers a unique in vitro method of metal nanoparticle synthesis by exogenously produced bacterial secondary metabolites, where direct contact between the toxic metal and biological resource material can be avoided. The biologically synthesized AgNPs are found to have anti-algal effects against two species of Chlorella (Chlorella vulgaris and Chlorella pyenoidosa), as indicated by zone of growth inhibition on algal culture plates. Further results exhibit concentration-dependent progressive inhibition of chlorophyll content in the algal cells by AgNPs, confirming the algicidal effect of AgNPs.

In-Situ Hydrothermal Synthesis of Bi-Bi2O2CO3 Heterojunction Photocatalyst with Enhanced Visible Light Photocatalytic Activity

Bismuth containing nanomaterials recently received increasing attention with respect to environmental applications because of their low cost, high stability and nontoxicity. In this work, Bi-Bi₂O₂CO₃ heterojunctions were fabricated by in-situ decoration of Bi nanoparticles on Bi₂O₂CO₃ nanosheets via a simple hydrothermal synthesis approach. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to confirm the morphology of the nanosheet-like heterostructure of the Bi-Bi₂O₂CO₃ composite. Detailed ultrafast electronic spectroscopy reveals that the in-situ decoration of Bi nanoparticles on Bi₂O₂CO₃ nanosheets exhibit a dramatically enhanced electron-hole pair separation rate, which results in an extraordinarily high photocatalytic activity for the degradation of a model organic dye, methylene blue (MB) under visible light illumination. Cycling experiments revealed a good photochemical stability of the Bi-Bi₂O₂CO₃ heterojunction under repeated irradiation. Photocurrent measurements further indicated that the heterojunction incredibly enhanced the charge generation and suppressed the charge recombination of photogenerated electron-hole pairs.

Enhanced Conversion Efficiencies in Dye-Sensitized Solar Cells Achieved through Self-Assembled Platinum(II) Metallacages

Two-component self-assembly supramolecular coordination complexes with particular photo-physical property, wherein unique donors are combined with a single metal acceptor, can be utilized for many applications including in photo-devices. In this communication, we described the synthesis and characterization of two-component self-assembly supramolecular coordination complexes (SCCs) bearing triazine and porphyrin faces with promising light-harvesting properties. These complexes were obtained from the self-assembly of a 90° Pt(II) acceptor with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPyT) or 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). The greatly improved conversion efficiencies of the dye-sensitized TiO₂ solar cells were 6.79 and 6.08 respectively, while these SCCs were introduced into the TiO₂ nanoparticle film photoanodes. In addition, the open circuit voltage (Voc) of dye-sensitized solar cells was also increased to 0.769 and 0.768 V, which could be ascribed to the inhibited interfacial charge recombination due to the addition of SCCs.

The role of Mott–Schottky heterojunctions in PtCo–Cu2ZnGeS4 as counter electrodes in dye-sensitized solar cells

Metastable wurtzstannite Cu₂ZnGeS₄ (CZGS) nanocrystals were synthesized via a hot-injection method and then used as matrixes to fabricate PtCo–CZGS heterostructured nano-particles.